This invention relates generally to mirrored databases used for online transaction processing (OLTP), and more particularly to the resynchronization of fixed page length tables on a mirror database.
Enterprises employ database systems comprising mirrored databases as a repository of the enterprise's stored data, and to support operational systems such as online transaction processing (OLTP). The databases generally have large sizes, store large volumes of data in fixed sized page length tables, and experience high numbers of online transactions that continually change existing pages and create new pages.
Mirrored databases comprise a primary database and a mirror database pair that are synchronized by redundantly writing the same data to both databases for backup and to assure high availability of the data if one of the databases fails (crashes). In the event of a crash, or loss of communications with a database, a mirror resynchronization process must be performed by the system to catch up lost changes on the mirror and new changes resulting from new online transactions while the mirror was down in order to restore the databases to a synchronized state. An important measure of database service availability is the time it takes for a mirror database to take over processing once a failure of the primary database has been detected. This time is referred to as the mean-time-to-repair (MTTR). Accordingly, it is important that resynchronization be performed timely so that the database has a very good repair-time to high-availability.
A problem for mirrored databases supporting OLTP is that mirror resynchronization can suffer from the chase-the-tail problem of trying to read and apply changes due to new ongoing online transactions at the same time the database is trying to mirror resynchronize, while falling behind an ever-increasing EOF (End of File) change tracking log that records changes due to new online transactions. The result is an inability to mirror resynchronize in a timely manner, which adversely affects high availability.
It is desirable to provide systems and methods that address these and other problems of timely resynchronization of mirrored databases while accommodating changes due to new online transactions to afford good repair time and high availability, and it is to these ends that the present invention is directed.